1. Field of the Invention
The present invention relates to a process for producing a porous metallic material to be used as an electrode substrate for use in an alkaline secondary battery principally such as a nickel-cadmium battery, a nickel-zinc battery or a nickel-hydrogen battery.
2. Description of the Prior Art
Storage batteries for use as various electric power sources include lead storage batteries and alkaline storage batteries. Among them, the alkaline storage batteries have been widely used in various portable apparatuses in the form of a miniature battery and in industrial applications in the form of a large-sized one, for example, because they can be expected to be high in reliability and can be miniaturized to be lightweight. In these alkaline storage batteries, materials for negative electrodes include zinc, iron, hydrogen, etc. in addition to cadmium. However, positive electrodes are nickel electrodes in almost all cases though an air electrode, a silver oxide electrode, etc. are partially adopted. Replacement of a sintered type for a pocket type has attained improvements in properties and enabled hermetic sealing thereof to widen the scope of uses thereof.
In a common powder-sintered type substrate, however, the strength thereof is greatly lowered when the porosity thereof is set to be at least 85%. Thus, there is a limit to filling it with an active material. Accordingly, there is a limit to increasing the capacity of a battery. In view of the above, a foamed substrate and a fibrous substrate have been adopted and put into practical use as substrates having a far higher porosity of at least 90% or the like in place of the sintered substrate. Processes for producing such a high-porosity porous metallic body substrate include a plating process as disclosed in Japanese Patent Laid-Open No. 174,484/1982, and a sintering process as disclosed in Japanese Patent Publication No. 17,554/1963 and the like. The plating process is a method wherein the skeletal surface of a foamed resin such as a urethane foam is coated with a carbon powder or the like to effect such a treatment thereof as to be rendered electrically conductive, and further subjected to Ni electrodeposition by electroplating, followed by burning out the foamed resin and the carbon to obtain a porous metallic material. On the other hand, according to the sintering process, the skeletal surface of a foamed resin such as a urethane foam is dipped in and coated with a slurry of a metal powder, followed by heating to sinter the metal powder.
As shown in the prior art, application of a porous metallic body to a battery plate substrate has made a great contribution to an increase in the capacity of a battery. In production of a porous metallic body according to the plating process as disclosed in Japanese Patent Laid-Open No. 174,484/1982, however, a porous resin core body must be coated with carbon to effect such a treatment thereof as to be rendered electrically conductive for electroplating. Carbon is necessary only in a step of production, but unnecessary in the porous metallic body because it is finally burnt out. Thus, coating the core body with carbon for such a treatment to make it electrically conductive not only entails an increase in the cost of a product, but also is believed to affect the quality of the product because of residual carbon. In this respect, an improvement has been desired. On the other hand, the production of a porous metallic body according to the sintering process as disclosed in Japanese Patent Publication No. 17,554/1963 does not fundamentally involve the above-mentioned problems, but can hardly secure desirable properties such as mechanical strength properties and electrical properties required of a battery plate substrate because dense sintering of a skeletal portion in the form of a porous body is difficult. On the other hand, Japanese Patent Publication No. 4,136/1994, directed to a process for producing a porous iron catalyst carrier, also discloses a method of obtaining a porous Fe body using an iron powder, an iron oxide powder, etc. According to this method, however, no properties required of a battery electrode substrate can be secured like in the foregoing cases, for example, because none other than a porous sintered body having a coarse skeletal portion can be obtained.